You can be pretty sure that when those two lost pilots in a little Cessna wandered out of Pennsylvania and into highly restricted air space near the Capitol and the White House last week, it wasn't just F-16 fighter jets and Blackhawk helicopters that were prepared to end their journey. Government men in black likely were posted atop key buildings with shoulder-fired missiles as well.

Such weaponry has been part of the US arsenal for decades. But just as many "bad guys" as "good guys" may be armed with MANPADS (man-portable air defense systems) these days, and some experts say that it would be far too easy for one of them to attack an American airliner. As a result, diplomats and engineers are scrambling to reduce the threat.

Intelligence sources estimate that as many as 27 terrorist and guerrilla groups around the world have such weapons, thanks to the hundreds provided by the CIA to anti-Soviet Afghan mujahideen in the 1980s, plus Russian and Chinese missiles sold on the black market. While much of Iraq's Saddam Hussein-era arsenal has been captured or destroyed, US officials have not been able to account for some 4,000 MANPADS there.

Earlier this year, the US and Russia agreed to destroy cheap shoulder-fired missiles that can bring down an airliner, and the US is providing technical support to other countries (including Nicaragua, Bosnia, Cambodia and Liberia) to destroy such antiaircraft missiles.

But thousands of cheap MANPADS remain in circulation - many of the estimated 500,000 to 750,000 produced since the 1970s and bearing such names as Stinger (US), Strela (Russian), Vanguard (Chinese), Blowpipe (British), and Mistral (French). At least 5,000 of those are estimated to be outside any government's control, and experts say just one could do great damage to the airline industry - as much as $70 billion in economic losses from a single attack.

"Although the loss of life from a single MANPADS attack would be considerably less than that caused by the 9/11 attacks - perhaps several hundred killed rather than thousands - the terror spread by such an attack could be just as profound," says Charles Peña, director of defense- policy studies at the Cato Institute in Washington, and author of the recent study "Flying the Unfriendly Skies."

Meanwhile, there is little doubt that international terrorists are armed with such weapons, which Mr. Peña points out "can be purchased on the military-arms black market for as little as $5,000."

"Al Qaeda and many other groups hostile to the United States have MANPADS and the ability to use them," the RAND Corporation reported earlier this year. "Al Qaeda ... has at least first-generation MANPADS, has the ability to move them about internationally, and has decided to employ MANPADS attacks as part of its terror campaign."

In 2002, Al Qaeda terrorists used two Russian-made Strela missiles in an unsuccessful attempt to bring down an Israeli-chartered airliner taking off from Mombasa, Kenya. In addition to Al Qaeda, nonstate groups confirmed to have shoulder-fired missiles include Chechen rebels, the Taliban, Tamil Tigers, Hizbullah, and FARC (the communist guerrillas in Colombia). As of 2003, according to the Congressional Research Service, there had been 35 recorded attacks against civilian aircraft, and 24 planes were shot down, killing more than 500 people.

These missiles typically home in on an aircraft engine's heat, travel faster than the speed of sound, and can reach up to three miles and 15,000 feet in altitude - easily covering the takeoff and landing profile when large aircraft are most vulnerable.

The question of protecting planes

Countermeasures to confuse attacking missiles are possible and in some cases available. These include decoy flares to draw off the incoming missiles and beams of light or lasers to jam a missile's homing device. Some US military aircraft and Israeli airliners are equipped with such devices. Last year, the Department of Homeland Security contracted with three firms to develop the technology for US airliners.

But outfitting and operating commercial aircraft with antimissile systems would be very expensive

"The problem with the laser jammers is their extensive operating and maintenance costs," says Victoria Samson, a researcher at the Center for Defense Information in Washington. "To fit out the entire fleet of [6,800] US civilian aircraft, it would cost over $11 billion initially, then run up to $2.1 billion of operating costs every year."

Given "significant uncertainties" in the cost and effectiveness of countermeasures, RAND recommends a go-slow approach in deployment of antimissile devices while research and development continues.

Not everyone agrees.

"It seems to me that whatever money we invest in protecting planes would be cheap in comparison to the cost of a shoulder-fired missile hitting planes later," says Rep. Steve Israel (D) of New York. "The cost of rebuilding the airline industry from the ground up is the more prohibitive one to me."

Mr. Israel sponsored unsuccessful legislation that would have retrofitted all US commercial aircraft with equipment to defend against shoulder-fired missiles. Until that happens, he says, civil aircraft remain vulnerable to terrorists.